With an increase in concern for the environmental issues, power storage devices such as secondary batteries and electric double layer capacitors used for power supply for hybrid vehicles and the like have been actively developed. In particular, a lithium ion battery and a lithium ion capacitor having high energy performance have attracted attention. The lithium ion battery, which is compact but can store a large amount of electricity, has been already mounted on a portable information terminal such as a mobile phone or a notebook personal computer, and has helped miniaturization of products.
The secondary battery and the electric double layer capacitor have a structure in which an electrolyte is provided between a positive electrode and a negative electrode. It is known that each of the positive electrode and negative electrode includes a current collector and an active material provided in contact with the current collector. For example, in a lithium ion battery, a compound capable of occluding and releasing lithium ions is used in electrodes as an active material, and an electrolyte is provided between the electrodes.
Various approaches have been taken to improve the characteristics of lithium ion batteries. Study of positive electrode active materials for lithium ion batteries is one example.
Compounds containing lithium and oxygen, and the like are known as a positive electrode active material of a lithium ion battery (see Patent Document 1).
In particular, lithium iron phosphate (LiFePO4) has attracted attention as a positive electrode active material. Lithium iron phosphate has advantages such as inexpensiveness. A lithium ion battery formed using as a positive electrode active material a compound which involves Fe2+/Fe3+ oxidation-reduction has the advantages of exhibiting high voltage (about 3.5 V), having favorable cycle characteristics, having higher energy density than a lithium ion battery formed using a compound such as lithium cobalt oxide (LiCoO2) or lithium nickel oxide (LiNiO2) as a positive electrode active material because of its theoretical capacity of about 170 mAhg−1, and the like.
However, when used as a positive electrode active material of a lithium ion battery, lithium iron phosphate has the problem of difficulty in achieving high output due to its slow lithium diffusion and low electron conductivity. In order to obtain a high-output lithium ion battery by increasing the area of contact between lithium iron phosphate and an electrolyte solution, there has been a report of a method for increasing a specific surface area by making lithium iron phosphate crystals into microparticles.
For example, it is reported in Non-Patent Document 1 that particles of lithium iron phosphate crystals synthesized by a hydrothermal method in a nitrogen atmosphere have sizes of approximately 300 nm to 500 nm.